A method of this kind is known from HINSHAW, D. A.; DOBBINS III, J. T.: Recent progress in noise reduction and scatter correction in dual-energy imaging, in: Proc. SPIE, 1995, Vol. 2432, pages 134-142. With said known method, scattered radiation correction is performed for each of the attenuation images recorded in different energy ranges by defining, for a given pixel, an empirically determined scattered radiation fraction as a function of the image value. The scattered radiation fraction determines the shape and width of a distribution function for the scattered radiation. The scatter contributions in adjacent pixels are calculated with the aid of the distribution function. The method is then repeated for further image values and the scatter contributions in the individual pixels are summed. Thus, a convolution of the image recorded by means of the detector device takes place with a distribution function, the width and shape of which depend on the image values of the attenuation image recorded by the detector device.
Scattered radiation correction is very important in dual X-ray absorptiometry.
More information about dual X-ray absorptiometry can be found in WARP, R. J.; DOBBINS, J. T.: Quantitative evaluation of noise reduction strategies in dual-energy imaging, in: Med. Phys. 30 (2), February 2003, pages 190-198.
In dual X-ray absorptiometry, the object to be examined, which is usually a patient, is irradiated by means of X-ray radiation in different energy ranges, whereby the dual X-ray absorptiometry can be effected by means of a single radiograph recording or a series of successively recorded radiographs. Typically, two radiographs are taken in succession.
In the first case, a dual detector having two different scintillation materials is used, the response characteristics of which have energy centers of mass lying as far apart from each other as possible. In the second case, succeeding images are recorded using as far as possible different X-ray spectra which, when X-ray tubes are used, can be generated by changes to the tube voltage by means of which the electrons are accelerated, or by selection of prefilters.
For each pixel of the recorded projection images it is possible to deduce the material composition in the beam path between the point X-ray source and the pixels from the attenuation characteristics in the different energy ranges. The projection images are also referred to in the following description as attenuation images. Furthermore, material composition is to be understood as meaning the mass per unit area of the different materials along the beam through the object to be examined.
In projection radiography using surface detectors, scattered radiation plays a significant role owing to the large solid angle recorded. In order to reduce the scattered radiation, anti-scatter grids are frequently used immediately above the detector input surface.
As a quantitative method, dual X-ray absorptiometry imposes higher requirements in terms of the accuracy of the measurement data than simple projection imaging as part of projection radiography. In spite of anti-scatter grids, the scattered radiation fraction falsifying the data may still be significant. For example, when images are recorded in the thoracic region, the air gap is usually very small, since the patient is positioned very close to the detector. As a result, in spite of anti-scatter grids, the intensity of the scattered radiation can still predominate over the primary intensity, especially in image regions with strong attenuation and at higher photon energies, corresponding to X-ray tube voltages in excess of 100 kV. Moreover, it is an empirical fact that the scattered radiation fractions are very different in the higher- and low-energy image data. All in all, in spite of anti-scatter grids, the presence of scattered radiation in dual X-ray absorptiometry can lead to unreliable and in some cases unusable results, to negative material thicknesses for example.
For this reason computational scattered radiation correction methods are necessary in dual X-ray absorptiometry, in addition to the use of anti-scatter grids.
It should be noted at this point that the scattered radiation is also referred to in the following as secondary radiation. In contrast, the unscattered radiation recorded by the detector is referred to as primary radiation. The sum of primary radiation and secondary radiation, which yields the measured image values, is referred to as the total radiation.
A metrological method for determining the scattered radiation in accordance with the beam-stop technique is known from FLOYD, C. B.; BAKER, J. A.; LO, J. Y.; RAVIN, C. E.: Posterior Beam-Stop Method for Scatter Fraction Measurement in Digital Radiography, in: Investigative Radiology February 1992, Vol. 27, pages 119-123. This method is suitable for applications in the laboratory using phantoms, but hardly for clinical operation.
Various computational methods for scattered radiation correction within the framework of computed tomography are known from ZELLERHOFF, M.; SCHOLZ, B.; RÜHNSCHOPF, E.-P.; BRUNNER, T.: Low contrast 3D reconstruction from C-arm data, in: Proceedings of SPIE. Medical Imaging, 2005, Vol. 5745, pages 646-655. However, the known computational methods are usually fairly complex and time-consuming.
There is therefore a need for comparatively simple correction methods by means of which the image quality can be significantly improved.